Aeron Biotechnology, Inc. proposes to develop a simple, economical, and rapid in vitro breast cancer cell bioassay that can be used to detect, distinguish between, and determine the bioactivity of estrogens and antiestrogens. To distinguish between estrogens and antiestrogens we will monitor differences in their molecular actions on estrogen receptor (ER) binding and activation, ER processing, end product induction, and cell proliferation in intact breast cancer cell lines. The proposed in vitro bioassay will have several commercial applications. It will provide a simple means by which a large panel of drugs, plant extracts, foodstuffs, etc. can be screened rapidly for estrogenic or antiestrogenic (ie, anticancer) activity. At the clinical level the bioassay will be useful for monitoring serum antiestrogen content and overall bioactivity in breast cancer patients receiving antiestrogen therapy.